Demands to increase storage capacity know no bounds, since technologies in information industry have been expanding and demands on storage, typically of images, have been increasingly made. Consequently, the size of memory is required to be minimized. It is now predicted that memory capacity of 100 gigabits per square inches can be achieved with a magnetic material size of about 30 nm (300 angstroms) in 2004, and memory capacity of 1000 gigabits per square inches can be achieved with a magnetic material size of about 10 nm (100 angstroms) in 2007.
To allow magnetic materials for storage to have a finer size and a higher density, the sensitivities of reproducing heads or magnetic sensors must be increased. The sensitivity is represented by MR ratio. Downsizing of reproducing heads have been achieved by increasing MR ratio of sensing elements constituting the reproducing heads.
Regarding the MR ratio, giant magnetoresistive (GMR) devices with MR ratio of 4% have been developed and brought into practice since around 1994. Those with MR ratio of about 10% are about to be released. At a storage density of 100 gigabits per square inches or more, however, the MR ratio of the GMR devices is still insufficient, and MR ratio of ten percent to several ten percent is required.
TMR devices having a MR ratio of 10% or more in development phase were achieved in 2000. The TMR devices could achieve a higher MR ratio that could not be achieved by the conventional GMR devices (Non-patent Document 1).
Such TMR devices can be applied not only as magnetic sensors but also as magnetic memories. IBM announced a joint development project for launching 256 megabit-MRAM on the market in 2004. Accordingly, the importance of the TMR devices is increasing.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-86863
[Non-patent Document 1] Ohashi et al., NEC “Low Resistance Tunnel Magnetoresistive Head”, IEEE Transaction on Magnetics, Vol. 36, No. 5, pp. 2549-2553, 2000
[Non-patent Document 2] M. Brown et al., App. Phys. 82 (2003) 233
[Non-patent Document 3] M. Kawasaki, Y. Tokura et al., J. Appl. Phys. Vol. 42 (2003) L369-L372
[Non-patent Document 4] U. Pustogowa et al., Phys. Rev. B49 (1994) 10031
[Non-patent Document 5] Th. Rasing et al., Phys. Rev. Lett. 74(1995) 3692 (J. Appl. Phys. 79 (1996), 6181)